Using high-speed III-N power switches involves balancing requirements for heat transfer, ease of assembly, and high-speed, low-inductance electrical interconnection. Conventional leaded power packages, such as any of the variations of the TO-220 package 100, which is illustrated in FIG. 1, can be used with III-N power switches. The combination of a metal mounting tab 102 and flexible copper leads 104, 106, and 108 permits attachment of the package to effective heat sinks in a variety of configurations. Connection to a PCB with conventional soldering techniques permits ease of manufacture.
Nonetheless, the package leads typically introduce undesirable inductance. Reduction in switching speed caused by this inductance may be an acceptable design compromise, but instability may still present a problem. Since a power switch can be a high-gain device, if allowed to operate in a linear mode, care should be taken that any oscillations due to parasitic resonances do not couple to a node where positive feedback may sustain or amplify the oscillations.
FIG. 2 is a circuit diagram of a half bridge circuit comprising a gate driver 202, a high side III-N transistor 204 coupled to a high voltage node 206, and a low side III-N transistor 208 coupled to a ground node 210. Two terminals 231 and 233 of the gate driver 202 are coupled to respective gates of the transistors 204 and 208, and two terminals 232 and 234 of the gate driver 202 are coupled to respective sources of the transistors 204 and 208, such that the gate driver is able to apply voltage signals to the gates of each of transistors 204 and 208 relative to their respective sources. An inductor 214 is coupled to the half bridge circuit at an output node 212.
In operation, the gate driver 202 can operate the transistors 204 and 208 in a constant-current mode (CCM), switching rated current at rated voltage. For example, the high voltage node 206 can provide a voltage of 400V or 600V or greater, and the III-N transistors can be configured with a rating to withstand the resulting high currents. Due to the inductance of the inductor 214, current flowing through the inductor 214 cannot change instantaneously.
To illustrate the operation of the half bridge, consider an example scenario where the gate driver 202 turns the high side transistor 204 on and turns the low side transistor 208 off. Current flows from the high voltage node 206, through the high side transistor 204, and through the output node 212 to the inductor 214. When the gate driver 202 turns the high side transistor 204 off, the inductance of inductor 214 drives the voltage at node 212 negative, which allows current to flow up through the low side transistor 208 even though it is off. If the half bridge is implemented using a conventional package, the undesirable inductance introduced by the package leads can cause significant ringing and oscillation related to transient current flowing through the circuit, which can interfere with a stable, efficient switching function.